Therefore we call 9 an odd number. We cannot take half of a natural number that is odd. We cannot take half of 9 people

The even numbers, then, are the numbers divisible by 2: 2, 4, 6, 8, and so on. The odd numbers are 1, 3, 5, 7, and so on.

If you do not know half of some even number, you can find it by decomposing it mentally into two even numbers whose half you do know. We saw how to do that in Section 1, Example 8.

Example 1. How much is half of 54?

Answer. Half of 54

=

Half of 50 + Half of 4

=

25 + 2

=

27.

As for odd numbers, note that every odd number is equal to an even number plus 1.

3 = 2 + 1.

5 = 4 + 1.

7 = 6 + 1.

9 = 8 + 1.

And so on.

Again, if a natural number is odd, then we cannot take half of it. Therefore, we will now assume that we are not necessarily dealing with natural numbers, and that 1 is now a continuous unit -- a unit we use to measure -- and that has a half: 1 minute, 1 centimeter, 1 pound, and so on. (Lesson 20.)